This invention relates to a measurement of impedance and, more particularly, to an apparatus for measuring impedance and a method used therein.
In the following description, term xe2x80x9cimpedancexe2x80x9d means an absolute value of the impedance of an electric circuit, a value of the real part, a value of the imaginary part and a ratio therebetween. A typical example of the apparatus for measuring impedance is disclosed in Japanese Patent Publication of Unexamined Application No. 61-266965. FIG. 1 illustrates the prior art apparatus for measuring impedance. Although the references are different, FIG. 1 is corresponding to FIG. 1 disclosed in the Japanese Patent Publication of Unexamined Application.
The prior art apparatus measures the impedance of a target circuit 100. The target circuit 100 is assumed to be a capacitive element, and has an admittance Y expressed as Y=G+jB where G is a conductance and B is susceptance. The prior art apparatus comprises a source of alternating current 102, a current-to-voltage converter 103, a phase discriminator 104, a phase shifter 105, a phase discriminator 106, a comparator 107, a switching unit 108 and an analog-to-digital converter 110. The source of alternating current 102 applies a voltage e to the target circuit, and a current ig flows out from the target circuit 100 into the current-to-voltage converter 103. The amount of current is expressed as e(G+jB). The current-to-voltage converter 103 converts the current ig to an output voltage ey, and the output voltage ey is equal to xe2x88x92Rxc3x97ig. The output voltage ey is applied to the phase discriminators 104 and 106.
The phase discriminator 104 multiplies the voltage ey by the output voltage e of the source of alternating current 102, and extracts the direct current component. The phase discriminator 104 outputs a dc voltage ea proportional to the conductance G. On the other hand, the other phase discriminator 106 multiplies the voltage ey by the output voltage of the phase shifter 105. The phase shifter 105 is supplied with the voltage e of the source of alternating current 102, and introduces a phase lag of 90 degrees between the voltage e and the output voltage. For this reason, the phase discriminator 106 outputs a dc voltage eb proportional to the susceptance B.
The switching unit 108 has two input nodes 109a and 109b. The dc voltage ea is applied to the input terminal 109a, and the other dc voltage eb is applied to the other input terminal 109b. The switching unit 108 selectively supplies the dc voltages ea and eb to the analog-to-digital converter 110, and the analog-to-digital converter 110 converts the dc voltages ea/eb to a digital signal.
The prior art measuring apparatus further comprises a microcomputer 111, an ac voltage-to-dc voltage converter 112, an analog-to-digital converter 113 and display units 114a/114b. The output voltage ey is supplied to the ac voltage-to-dc voltage converter 112, and the ac voltage-to-dc voltage converter 112 produces a dc voltage from the output voltage ey. The dc voltage is proportional to absolute value of the admittance Y. The dc voltage is supplied to the analog-to-digital converter 113, and is converted to a digital signal.
The analog-to-digital converters 110 and 113 are connected to the microcomputer 111. The microcomputer 111 calculates the conductance G and the susceptance B on the basis of the digital signal supplied from the analog-to-digital converter 110, and the displays 114a/114b indicate the conductance G, the susceptance B, respectively. The microcomputer calculates the absolute value of the admittance on the basis of the digital signal supplied from the analog-to-digital converter 113.
The comparator 107 behaves as follows. The dc voltage ea is compared with the dc voltage eb. If the admittance Y is much greater than the conductance G, i.e., Y greater than  greater than G, the susceptance B is expressed as
B={square root over ( )}(Y2xe2x88x92G2)≈Y
For this reason, the microcomputer 111 ignores the digital signal converted from the dc voltage eb, and calculates the susceptance B on the basis of the digital signal supplied from the analog-to-digital converter 113.
On the other hand, when the admittance Y is much greater than the susceptance B, i.e., Y greater than  greater than B, the conductance G is expressed as
G={square root over ( )}(Y2xe2x88x92B2)≈Y
For this reason, the microcomputer 111 ignores the digital signal converted from the dc voltage ea, and calculates the conductance G on the basis of the digital signal supplied from the analog-to-digital converter 113. This is because of the fact that the ac voltage-to-dc voltage converter 112 is much higher in accuracy than the phase discriminators 104/106. In fact, the error introduced by the phase discriminators 104/106 is of the order of 0.1 to 0.2 percent. On the other hand, the error introduced by the ac voltage-to-dc voltage converter 112 is of the order of 0.01 percent. Thus, the microcomputer 111 gives the priority to the digital signal supplied from the ac voltage-to-dc voltage converter 112 through the analog-to-digital converter 113, and enhances the accuracy.
The Japanese Patent Publication of Unexamined Application further discloses an apparatus for measuring an impedance of an inductive element. The resistance R, the reactance X and the impedance Z are measured in a similar manner to that described hereinbefore. The microcomputer also gives the priority to the digital signal converted from the dc voltage representative of the impedance Z, and calculates the resistance R or the reactance X on the basis of the digital signal under the condition of Z greater than  greater than X or Z greater than  greater than R.
Although the priority given to the ac voltage-to-dc voltage converter 112 fairly improves the accuracy of the measurement, i.e., either conductance or susceptance under the conditions of Y greater than  greater than G or Y greater than  greater than B, the measurement still contains the susceptance or the conductance calculated on the basis the digital signal converted from the dc voltage eb or ea. When the admittance Y is not much greater than the conductance G and the susceptance B, both of the conductance G and the susceptance B are calculated on the basis of the digital signals converted from the dc voltages ea and eb. Thus, the prior art apparatus still has a problem in the accuracy of the measurement. This is the first problem inherent in the prior art apparatus.
The second problem is a noise component due to the dc offset voltage in the phase discriminators 104/106. The prior art apparatus is connected to various kinds of electric circuits 105, and the phase discriminators 104 and 106 require a dc amplification for the dynamic range. The dc voltages ea and eb contain the dc offset voltage, and the dc offset voltage is transferred to the digital signal through the analog-to-digital conversion. Thus, the digital signal contains the noise component, and the noise component deteriorates the measurement. This is the second problem inherent in the prior art apparatus.
The third problem is the phase shifter 105. Although the phase shifter 105 targets the phase lag for 90 degrees, the phase shifter can not shift the output voltage e by 90 degrees at all times. This means that the analog multiplication is not accurate.
The same problems are encountered in the prior art apparatus used for an inductive element.
It is therefore an important object of the present invention to provide an apparatus for accurately measuring an impedance.
It is also an important object of the present invention to provide a method for accurately measuring an impedance.
To accomplish the object, the present invention proposes to digitize a signal processing for determining an impedance.
In accordance with one aspect of the present invention, there is provided an apparatus for measuring an impedance of an object comprising a port connected to the object, a periodic signal generator connected to the port and supplying a first analog signal periodically varied and produced from a first digital signal through the port to the object for generating a second analog signal varied due to the impedance, a digital signal generator producing a second digital signal from the second analog signal and a data processor connected to the periodic signal generator and the digital signal generator and supplied with the first digital signal and the second digital signal for determining the impedance.
In accordance with another aspect of the present invention, there is provided a method for measuring an impedance of an object comprising the steps of a) generating a first analog signal from a first digital signal, b) supplying the first analog signal to the object for producing a second analog signal varied due to the impedance, c) converting the second analog signal to a second digital signal and d) determining the impedance through a digital processing on the first digital signal and the second digital signal.